Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant material is injected into the wellbore at high pressures. Once natural reservoir pressures are exceeded, the fluid induces fractures in the formation and proppant is deposited in the fracture, where it remains after the treatment is completed. The proppant material serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture. Because fractured well productivity depends on the ability of a fracture to conduct fluids from a formation to a wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment.
The solid proppant materials are typically mixed with a gelled carrier fluid (e.g., aqueous-based fluid such as gelled brine) and injected into the well in order to create the high conductivity channel. Gelling agents for proppant carrier fluids may provide a source of proppant pack and/or formation damage, and settling of proppant may interfere with proper placement downhole. Formation damage may also be caused by gelled carrier fluids used to place particulates downhole for purposes such as for sand control, e.g., gravel packs, frac packs, etc.
Formulation of gelled carrier fluids usually requires equipment and mixing steps designed for this purpose. At the time of proppant addition, the carrier fluid exhibits poor solid suspending properties and vigorous agitation is required to prevent gravity segregation of the solids. A typical equipment set-up for hydraulic fracturing is set forth in FIG. 1, wherein the carrier fluid is delivered either from one or more pre-gelled tanks or customized hydration units, 10A, 10B and 10C. The carrier fluid is mixed in mixing unit 40 with buffers, breakers, surfactants and other additives which may be required during treatment. The proppant is delivered from one or more storage bins or silos, 20A and 20B, by gravity and added to the fluid by way of conveyors or augers, 30. The operation of combining the proppant with the fluid involves the use of slurry blender 50, a relatively sophisticated and costly piece of equipment. The slurry blender homogenizes the mix of proppant and carrier fluid and allows the addition of viscosifying enhancing agents, such as crosslinking agents, thus improving proppant transport. Further, it feeds at least one high pressure pump, shown as a series of pumps 60A, 60B and 60C, which are used to inject the proppant slurry into the wellhead 70. The need to “ramp” or step-up the concentration of proppant, as the operation proceeds, requires considerable operator expertise and/or requires the use of an array of process control equipment to enable accurate proportioning of all the components at various rates. Any operational failure, such as tub overflow, improper amount of viscosity enhancer, breaker, etc., jeopardizes the operation or its results.
Attempts have been made with conventional proppants to obtain pumpable formulations for use on the fly. Unfortunately, such formulations require a high degree of fluid gellation to maintain suspension of the heavy particles. Even with heavy gellation, such suspensions are further subject to particle settling within a matter of hours, particularly in the presence of vibration. This necessitates well defined mixing capabilities in order to homogeneously re-suspend the proppants in high viscosity suspension gels on-site. Significant costs are further incurred for the chemicals, equipment and processing time in order to gel the carrier fluid. Pumpable suspensions which do not exhibit particle settling are therefore desired.